Process of forming electrical connections between conductive layers using thermosonic wire bonded bump vias and thick film techniques

ABSTRACT

The process of the present invention allows interconnecting one conductive layer to another in a Multichip Module. The process uses bumps formed by wire bonding, or any other means of forming bumps of 0.002&#34;-0.004&#34; height with comparable diameter. These bumps create an opening through dielectric insulating layers, thereby allowing one conductive layer to be electrically connected to the next conductive layer.

FIELD OF THE INVENTION

The present invention relates to a process used to manufacture highdensity interconnect substrates. These substrates are used in a varietyof electronic applications, including telecommunications, automotive andindustrial controls, military electronics, and computers and businessequipment.

BACKGROUND OF THE INVENTION

Vias are used to interconnected different layers of Multichip Modules(MCM). Numerous processes have been used to manufacture high densityinterconnect vias. These vias are critical in the manufacture of MCM,which is a growing packaging technology in the electronic field.

Prior to the present invention, to form vias with a thick film additiveprocess, screen printing is used. A dielectric ink is screen printedonto the substrate with openings for the vias. The dielectric ink isfired at high temperatures, and a conductive ink is used to fill thevias before subsequent layers are added. However, the rheology of thedielectric ink is such that its flow causes small via openings of lessthan 0.008" to close.

Photo-imaging and "diffusion patterning" processes have been developedto address this problem, but these are both subtractive processes, i.e.,material must be washed away. This creates a need for expensive capitalequipment for the imaging, developing, etching, and dealing with wastecreated. Additionally, neither process has been used in production on alarge scale.

High density interconnects can be formed using thin film sputtering andchemical vapor deposition processes, but these too require capitalequipment costing from hundreds of thousands to millions of dollars.Also, thin film processes are unproven in the creation of thickdielectrics needed for reliability.

Therefore it is the objective of the present invention to provide aneconomical, safe, and reliable means of interconnecting layers on anMCM.

SUMMARY OF THE INVENTION

In order to accomplish the object of the present invention there isprovided a method of electrically connecting a first conductive layerwith a second conductive layer. First, the first conductive layer isformed on the substrate. Next, conductive bumps are bonded to the firstconductive layer, the conductive bumps having a height between 0.002"and 0.004". The first conductive layer is coated with a nonconductivelayer, the nonconductive layer having a thickness less than the heightof the conductive bumps. Finally, the second conductive layer isdeposited over the nonconductive layer making electrical contact withthe conductive bumps thereby electrically connecting the secondconductive layer to the first conductive layer.

DESCRIPTION OF THE DRAWINGS

A better understanding of the invention may be had from theconsideration of the following detailed description taken in conjunctionwith the accompanying drawings, in which:

FIGS. 1A and 1B show simplified diagrams of the first step of thepresent invention.

FIGS. 2A and 2B show simplified diagrams of the second step of thepresent invention.

FIGS. 3A and 3B show simplified diagrams of results of using the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The process of the present invention allows interconnecting oneconductive layer to another in a Multichip Module. The process usesbumps formed by wire bonding, or any other means of forming bumps of0.002"-0.004" height with comparable diameter. These bumps create anopening through dielectric insulating layers, thereby allowing oneconductive layer to be electrically connected to the next conductivelayer.

Referring to FIGS. 1, 2 and 3. Note: FIGS. 1, 2, and 3, are meant toonly show the process of the present invention; the end product, asshown in FIG. 3, is not intended to implement any particular function.The method of forming small vias is accomplished as follows:

First, the bottom conductive layer 3 is deposited onto the substratematerial 1. The material used for the first conductor is not criticaland may be gold, copper, nickel, or other air-firable thick film cermet.The deposition process also is not critical and may be thick film, thinfilm, or plating. Depending on the deposition process, the substrate 1and first conductive layer 3 may need to be fired to cure the cermet.

Next, a thermosonic wire bonder is used to create 0.002" to 0.004" ball"bumps" 2 on the first layer. The bumps are formed on pads that are tobe routed up to the next layer. The process of thermosonic wire bondingis known in the art.

A dielectric thick film paste is next deposited on top of the firstconductive layer either through screen, flow coating, or other means. Ifscreen printing is used it should have a combination of direct andindirect emulsions to prevent the bumps 2 from being coated. Thedeposited coating is 0.002"-0.003" in height in the wet stage,0.0003"-0.0007" below the height of the bumps. The coating flows untilit surrounds the bumps. This is then fired to create a dense insulatingfilm 4, with the bumps 2 providing a conductive path to the firstconductive layer 3 through the insulting film 4.

Finally, the second conductive layer 5 is deposited over the denseinsulating film 4 using either thin or thick film processing. Thus thesecond conductive layer 5 is electrically connected to the firstconductive layer 3 through the bumps 2.

The previous steps may be repeated depending on the number of conductorlayers required.

To summarize, first a conductive layer is deposited. Next conductivebumps are formed on the first conductive layer. Over the conductivelayer a nonconductive layer with a thickness substantially equal to, butslightly less than, the heights of the conductive bumps is applied.Another conductive layer can be deposited over the nonconductive layer,making electrical contact to the lower layer through the exposed bumps.This process may be repeated as needed.

Although the preferred embodiment of the invention has been illustrated,and that form described, it is readily apparent to those skilled in theart that various modifications may be made therein without departingfrom the spirit of the invention or from the scope of the appendedclaims.

What is claimed is:
 1. A method of electrically connecting a firstconductive layer with a second conductive layer using a thick filmtechnique, said first conductive layer being on a substrate, said methodcomprising the steps of:forming said first conductive layer on saidsubstrate; bonding conductive bumps to said first conductive layer, saidconductive bumps having a height, said step of bonding beingaccomplished by use of a thermosonic wire bonder; coating said firstconductive layer with a nonconductive layer, said nonconductive layerhaving a thickness less than said height of said conductive bumps, saidnonconductive layer coating around said conductive bumps; and depositingsaid second conductive layer over said nonconductive layer, said secondconductive layer making electrical contact with said conductive bumpsthereby electrically connecting said second conductive layer to saidfirst conductive layer.
 2. A method of electrically connecting a firstconductive layer with a second conductive layer as claimed in claim 1,wherein said step of forming being accomplished by screen printing saidfirst conductive layer onto said substrate.
 3. A method of electricallyconnecting a first conductive layer with a second conductive layer asclaimed in claim 1, wherein said step of coating being accomplished byscreen printing said nonconductive layer onto said first conductivelayer.
 4. A method of electrically connecting a first conductive layerwith a second conductive layer as claimed in claim 1, wherein said stepof depositing being accomplished by screen printing said secondconductive layer onto said nonconductive layer.
 5. A method ofelectrically connecting a first conductive layer with a secondconductive layer as claimed in claim 1, wherein said height of saidconductive bumps being between 0.002 and 0.004 inches.
 6. A method ofelectrically connecting a first conductive layer with a secondconductive layer as claimed in claim 1, wherein said conductive bumpsbeing formed from a gold wire.
 7. A method of electrically connecting afirst conductive layer with a second conductive layer, said firstconductive layer being on a substrate, said method comprising the stepsof:first screen printing said first conductive layer on said substrate;bonding conductive bumps to said first conductive layer, said conductivebumps having a height, said step of bonding being accomplished by use ofa thermosonic wire bonder; second screen printing said first conductivelayer with a nonconductive layer, said nonconductive layer having athickness less than said height of said conductive bumps; and thirdscreen printing said second conductive layer over said nonconductivelayer, said second conductive layer making electrical contact with saidconductive bumps thereby electrically connecting said second conductivelayer to said first conductive layer.
 8. A method of electricallyconnecting a first conductive layer with a second conductive layer asclaimed in claim 7, wherein said height of said conductive bumps beingbetween 0.002 and 0.004 inches.
 9. A method of electrically connecting afirst conductive layer with a second conductive layer as claimed inclaim 7, wherein said conductive bumps being formed from a gold wire.10. A method of electrically connecting a first conductive layer with asecond conductive layer using a thick film technique, said methodcomprising the steps of:forming said first conductive layer; bondingconductive bumps to said first conductive layer, said conductive bumpshaving a height, said step of bonding being accomplished by use of athermosonic wire bonder; coating said first conductive layer with anonconductive layer, said nonconductive layer having a thickness lessthan said height of said conductive bumps; and depositing said secondconductive layer over said nonconductive layer, said second conductivelayer making electrical contact with said conductive bumps therebyelectrically connecting said second conductive layer to said firstconductive layer.